As the communication technology is gravitating toward the millimeter wave (mmWave) technology, beam forming and multi-input multi-output (MIMO) technology will be featured in the upcoming 5G communication system for which a greater number of mobile devices as well as an explosive growth of the amount of data transmission will be anticipated. In order to solve the problem of frequency selective fading in MIMO systems, researchers have tried to using orthogonal frequency division multiplexing (OFDM). Meanwhile, the technological upgrade into 5G will bring about an increase of the system bandwidth, an increase of antenna operational frequency, an increase of the number of antennas, and so forth. The increase of the system bandwidth will bring about an increase of sampling frequency.
Although an antenna would transmit and receive signals in the radio frequency (RF) or mmWave frequency, the signals would need to be converted into digital signals in order to be used by a modern digital communication system. In order to convert between the analog domain and the digital domain, an analog-to-digital converter (ADC) has been used to convert from an analog signal format into a digital signal format. Similarly, a digital-to-analog converter (DAC) is used to convert from a digital signal format to an analog signal format.
Under a high sampling frequency, the design of the ADC and DAC would be quite challenging. Also, since MIMO would demand a large quantity of antennas, the number of ADCs/DACs to be used will also increase as one antenna would typically require one ADC or DAC. However, the increased number of ADC or DAC will bring about problems in the future when the 5G communication system is implemented. For example, in order for a receiver to deduce the impact of quantization error, a high resolution ADC (HADC). If a high number of HADCs is used in the 5G communication system in conjunction with the expected high transmission bandwidth, the sampling frequency of the HADC will be quite high. This would mean that the power consumption of the HADC will be high, and thus the battery of a mobile phone could be drained more quickly. Also, the increase number of HADCs would mean that the cost of a communication apparatus would be quite high as the number of HADC would need to proportionally match the number of MIMO antennas and thus would increase the overall cost of a receiver.
FIG. 1A is a line graph which illustrates the throughput versus power consumption of various commercial ADCs. It could be deduced from FIG. 1A that with the number of bits being equal, the increase of power consumption would be nearly proportional to the increase of sampling frequency. FIG. 1B is a line graph which illustrates the resolution versus power consumption of various commercial ADCs. It could be deduced from FIG. 1B shows that with the sampling speed being equal, the cost of HADC would increase with the sampling frequency. Also in a typical communication system, the resolution of an ADC is an important factor that affects the overall system performance as an ADC could be a resource of unfavorable noise figures (NF). FIG. 1C shows the bit error rate versus SNR between a high resolution channel and a low-resolution channel. FIG. 1C would show that an ADC could be a source of quantization error.
In the foreseeable future, a 5G communication system would need to process a very large frequency width and thus the sampling rate of ADCs will be quite high. As the sampling frequency becomes higher, the power of the ADCs will increase. In order to compensate for the transmission path loss of an mmWave communication system, the use of large antenna array cause a large number of ADCs to be required. However, it is nearly impossible currently to design an ADC hardware having both a high resolution and a high sampling rate. Even if such design is feasible, the ADC will be quite cost prohibitive.
FIG. 1D illustrates a SNR budget diagram of a hypothetical ADC. In general, an ADC in a RF/mmWave receiver would need to meet a specific SNR budget in order for the overall communication system to achieve a reasonable system performance. Although a high resolution ADC (HADC) could be used to achieve a reasonable system performance, the overall cost and power consumption as the result of the increase of the number of antennas would render the proliferation of HADCs unfeasible. Currently there is hardly any solution that uses low-resolution ADCs to achieve the same system performance as HADCs.